Module 3 - Section 3 - Conservation Of Energy And Efficiency

Question 1

What is kinetic energy ?

Answer 1

Kinetic energy is the energy an object has due to its movement

Question 2

Whats the equation for kinetic energy ?

Answer 2

Ek = 1/2 mv*2

m = mass
v = velocity

Question 3

How is the equation for kinetic energy derived ?

Answer 3

The change in kinetic energy = work done - so Ek = Fx - this is because work done is the amount of energy transferred from one form to another when a force causes a movement of some sort - and a ‘change’ in kinetic energy means that an energy has been transferred from one form to another because energy is conserved and can’t just disappear so that means its been transferred - also Ek only happens due to some sort of motion therefore a change in Ek means that a energy has been transferred to another form because a force caused a movement - change in Ek = Fx

Then what’s F —> F = ma —> Whats a ? —> a = v-u/t —> Whats x ? —> x or s = (u+v) / 2 x t

So then we put these together Ek = max = m x (v-u) / t x (u+v) / 2 x t
Then cancel out the ‘t’ and then expand the brackets and you will get Ek = 1/2mv2 - 1/2mu2

And the Ek of an object is the Ek compared to when the object is at rest and so when you substitute u=0 into the equation you get left with Ek=1/2mv*2

Question 4

Whats gravitational potential energy ?

Answer 4

Gravitational potential energy ( Ep ) is the energy an object has due to its position in a gravitational field - the greater the height of the object the greater the gpe of the object

Question 5

Whats the equation for gpe ?

Answer 5

G.P.E = mgh
m = mass
g = gravitational field strength
h = change in height

Question 6

Whats the derivation of the equation gpe=mgh ?

Answer 6

The gravitational potential energy gained by an object is = to the work done in moving the object a distance ‘h’ upwards —> so you get Ep = W = F x h

And the force that work is done against is the force of gravity —> which is equal to mg

So Ep = mgh

Question 7

What is the principle of conservation of energy ?

Answer 7

Energy cannot be created or destroyed - energy can only be transferred from one form to another but the total amount of energy in a closed system will not changed. So the energy can be transferred usefully and can be dissipated into energy stores such as heat and sound

Total energy in = total energy out

Question 8

What is Efficiency ?

Answer 8

Efficiency is the ratio of total energy input to useful energy output

It’s a measure of how well a device converts the energy you put in, into the energy you want it to give out ( useful energy )

So a device that wastes a lot of energy as heat and sound has a very low efficiency

Question 9

Whats the equation for efficiency ?

Answer 9

Efficiency = useful output energy / total input energy x 100

Question 10

Why can a device never be 100% efficient ?

Answer 10

Because some energy is always lost as heat